Balsa wood footing for lng tanks

ABSTRACT

A foundation for cryogenic storage tanks is shown. It is comprised of a balsa wood, softwood or hardwood footing positioned between a floor and subfloor of the tank. The footing is comprised of select blocks of wood glued to one another, their longitudinal axes vertically positioned. In a preferred embodiment the footing may be comprised of a three dimensional assembly of blocks glued to one another, their longitudinal axes positioned alternately in the vertical, radial and tangential directions. Longitudinal and transverse restraining bars hold the wood in place. Also shown is a balsa wood or softwood layer between the wall and the floor which acts as a load equalizer and sliding bearing for the wall.

United States atent [191 Wichman et al.

[111 emo [45] Jan. 14,1975

[ 1 BALSA WOOD FOOTING FOR LNG TANKS [73] Assignee: Preload TechnologyInc Garden City, N.Y.

[22] Filed: Mar. 19, 1973 [21] Appl. No.: 342,591

[52] US. Cl 220/18, 52/249, 52/573, 220/9 LG, 220/69 [51] Int. Cl 1365!!87/24, E04b 1/68 [58] Field of Search 220/9 A, 9 LG, l5, 18, 220/69, 11;52/245-249, 292, 293, 573, 274

[56] References Cited UNITED STATES PATENTS 2,495,798 1/1950 Wissmiller220/15 2,520,883 8/1950 Kornemann et al 220/9 LG 2,563,118 8/1951Jackson 220/15 2,911,125 11/1959 Dasker.... 220/11 2,954,892 10/1960Dosker 220/11 3,067,901 12/1962 Van Geem et al.... 217/65 3,092,9336/1963 Closner et al 52/249 Smith et a1. 220/9 LG 3,325,037 6/1967 Kohnet a1.

3,514,913 6/1970 Nelson 3,606,067 9/1971 Jones 220/15 PrimaryExaminerWilliam 1. Price Assistant Examiner-Stephen Marcus Attorney,Agent, or FirmCurtis, Morris & Safford [57] ABSTRACT A foundation forcryogenic storage tanks is shown. It is comprised of a balsa wood,softwood or hardwood footing positioned between a floor and subfloor ofthe tank. The footing is comprised of select blocks of wood glued to oneanother, their longitudinal axes vertically positioned. In a preferredembodiment the footing may be comprised of a three dimensional assemblyof blocks glued to one another, their longitudinal axes positionedalternately in the vertical, radial and tangential directions.Longitudinal and transverse restraining bars hold the wood in place.Alsoshown is a balsa wood or softwood layer between the wall and thefloor which acts as a load equalizer and sliding bearing for the wall.

7 Claims, 6 Drawing Figures SHEET 10F 3 Pmmmmmm FIG. 5

PATENTEDJANWQYS SHEET 20E 3 BALSA WOOD FOOTING FOR LNG TANKS Thisinvention relates broadly to cryogenic tanks for storage of liquidmethane and other liquified gases. More specifically, this inventionrelates to the novel construction of foundations with unique structuraland thermal properties for such cryogenic tanks.

In recent years the demand for liquid natural gas has substantiallyincreased and the art is faced with the need for providing larger andlarger tanks for containment of this liquid. The ever-present andincreasing need for safety in the design of such tanks has also creatednumerous problems in their construction and has presented the art with anumber of related problems in material selection and in tank design.Special problems arise in the design of prestressed concrete cryogenictanks. In these tanks, thermal gradients which cause differentialmovements of the walls and floor create critical stress conditions bothbefore and after service conditions are obtained, giving rise to anumber of unique design problems.

A very basic and important part of the cryogenic tank is the foundationstructure. The foundation must be able to withstand very considerablecompressive loads that are imposed on it by the wall structure.Furthermore, the foundation structure, particularly when the walls areprestressed, must be able to absorb multidirectional stresses due torelative movement of the walls and floors of the tank. Of equalimportance, the foundation materials must have good thermal insulatingproperties so that the foundation does not comprise an excessive heatsource for the tank. Heretofore the foundation structures which havebeen used in cryogenic tank design have been relatively expensive andhave not provided the multiple functions that are required under thevaried conditions of stress and temperature.

It is thus the primary object of this invention to provide an economicaland easily constructed foundation for cryogenic tanks.

It is a further and related object of this invention to provide afoundation structure which is fully able to absorb the very considerablecompressive stresses which are encountered where a high concrete orsteel wall is erected over the foundation.

It is still a further and related object of this invention to provide afoundation structure which is able to absorb and redistribute unevenstresses which may develop due'to inaccuracies in the construction ofthe tank or due to uneven cool-down or warm-up of the tank during itsservice.

It is a related object of this invention to provide a foundation whichcan withstand the varying forces imposed upon it in vertical, radial andhorizontal directions.

It is still a further and important object of this invention to providea foundation structure which can absorb vertical bending moments whichmay be imposed upon it by hydrostatic forces, thermal forces orprestressing of the tank wall.

A related object of this invention is to provide a foundation which hasa substantial degree of structural integrity under the influence ofearthquakes or other ground shock forces.

It is yet a further object of this invention to provide a foundationwhich has a high degree of thermal insulating property.

It is yet a further and also important object of this invention toprovide a foundation which permits some relative movement of the walland foundation, as occurs during the prestressing, shrinkage, creep,filling and emptying motions of a prestressed concrete tank.

These and other objects of this invention are obtained in an improvedfoundation for cryogenic tanks which is comprised of balsa, softwood orhardwood, oriented with the grain parallel to the load on thefoundation. Since wood is difficult to obtain in very large uniformblocks, it hasbeen found that good results are obtained if thefoundation blocks are themselves comprised ofa plurality of selectblocks which are attached to one another with a cryogenically compatibleadhesive material. In a particularly satisfactory foundation design, thefoundation comprises a subfloor, which may be of steel, and a tank floorlocated thereabove which is of cryogenic steel, and positionedtherebetween a foundation of wood with its grain oriented vertically,i.e. parallel to the tank wall. In a desirable variant, the blocks maybe arranged in a three dimensional assembly comprising blocks havingtheir grain oriented in the vertical, radial and tangential directions.

The tank wall is positioned above the tank floor and desirably isseparated from the tank floor by a thin layer of balsa wood or softwoodhaving its grain oriented horizontally. The thin layer of balsa wood orsoftwood has a cryogenic lubricant coating on its lower surface, and atits upper surface is attached by means of cryogenic adhesive to thelower surface of the wall. This balsa or softwood layer helps tosmoothen out irregularities inherent in large structures, and acts as asliding bearing between the wall and the floor permitting inward radialmovement of the wall due to prestressing or other forces which may beimposed upon it.

In the preferred embodiment of the invention, which is disclosed inconnection with the drawings, the wood foundation is retained inposition by circumferentially running members, such as angle irons,which are welded to the subfloor of the foundation. Desirably,restraining shear key bars are provided which are positioned atintervals transversely to the course of the foundation. These areattached to the tank floor and to the tank subfloor extending downwardlyand upwardly, respectively. These shear bars prevent relative tangentialmovement of the foundation and the wall. They do permit some radialmovement, however, as aforedescribed.

To protect the wood in the foundation from physical damage as well aswater absorption, it is desirable to coat all surfaces thereof withfiberglass. In constructions where the upper thin horizontal-grain layeris not used, it is preferable to cover the upper surface of the blockscomprising the footing with a teflon coated glass fiber sheet. Inanother preferred embodiment, the wood' in the foundation is sandwichedbetween panels of plywood.

The invention is further described in connection with the followingdrawings.

FIG. 1 is a side-elevation view of a foundation structure according tothe present invention;

FIG. 2 is an expanded side elevation view showing the essential detailof the foundation structure and of a balsa wood layer which acts as asliding bearing and bearing pressure equalizer;

FIG. 3 is a plan view in section of the foundation of FIG. 2, takenalong lines 3-3 of FIG. 2;

FIG. 4 is a plan view in section of the foundation of FIG. 2, takenalong lines 4-4 of FIG. 2;

FIG. 5 is a side elevation view of a portion of a foundationrepresenting a preferred embodiment of this invention; and

FIG. 6 is a side elevation view of a portion of a foundationrepresenting another preferred embodiment of this invention.

In FIG. 1 reference numeral 10 refers generally to a cryogenic tankstructure and more specifically to the foundation area thereof.Reference numeral 12 refers to a steel subfloor positioned above theconcrete foundation 14. Reference numeral 16 refers to a primary steelfloor of the cyrogenic tank. Steel subfloor l2 and steel floor 16 areseparated by a layer of insulation 20 which may be plastic foam or othersuitable insulating material. Between subfloor 12 and floor 16, at theedge of tank 10, is a foundation structure, referred to generally byreference numeral 22, which includes the wood supporting members andvarious restraining members, all as further described in connection withFIG. 2. Above floor 16 is a layer of balsa 24 which acts as a loaddistributor and as a sliding bearing between floor I6 and concrete wall26 which is located immediately thereabove. A steel liner 28 is providedat the exterior of wall 26.

With reference to FIGS. 2, 3 and 4, reference numeral 30 refers to awood footing or foundation wherein the grain of the wood is positionedvertically, i.e. parallel with the load imposed upon it by the weight ofof concrete wall 26. The wood footing may be a single block of wood,however, preferably and typically it will be an assembly of wood blocksattached to one another by means of a cryogenic adhesive. Each of thecomponent blocks should be sterilized and kiln dried to a controlledmoisture content of between 5 percent and percent. The blocks should berelatively free of holes and knots or decay which would affect theirstructural properties. The blocks are then typically laminated underpressure to one another with an adhesive such as phenol resorcinol,which is stable under the cryogenic temperatures of liquid natural gasstorage, i.e. stable at temperatures of 25 to 300F.

The wood footing which is obtained has a number of unique propertieswhen used for the service for which it is herein intended. This isespecially true when the wood used is balsa. When very high insulatingvalue is needed, loads are moderate, and a light weight wood ispreferred, balsa is most suitable. Its weight ranges between 6 and 16lbs. per cubic foot. The balsa wood footing is orthotropic having threeprincipal axes: a longitudinal, a radial and a tangential axis. Thecompressive strength is greatest in the longitudinal or end graindirection and its compressive strength in that direction is nearly 10times that in either the radial or tangential flat grain direction. Forthis reason, the balsa wood is used with its grain parallel to theheight of the wall which is erected above it. The footing is thusextremely difficult to deform due to the relatively small percentage ofsolid matter in the wood structure, and the wood has significantelasto-plasticor rebound properties under the different loads which maybe imposed on it. Composite blocks, which are obtained by gluing smallerselect blocks to one another, have strength properties which aresuperior to those of the individual component blocks themselves. Thefootings are able to redistribute excessive local loads which may occurand are able to rebound after high intensity loading is removed.Furthermore, the blocks have high resistance to splitting. The thermalqualities of the footings are also beneficial in that the porous natureof the balsa wood provides a relatively high and consistent insulatingvalue to avoid heat leak into the cryogenic tanks. Furthermore, thebalsa wood has good strength characteristics at cryogenic temperatures,its strength being 50 percent greater at 260 than at F.

To protect the balsa blocks from moisture and from abuse in handling andconstruction steps, the blocks are desirably fiberglassed on allexterior surfaces. This material is applied togetherwith a layer of 2oz. per square foot of fiberglass mat. A coating thickness of 60 milshas been found to suffice to protect the balsa wood. If desired, anultra-violet stabilizer can be added to the resin.

Before insulating, the top faces of the footings are desirably coveredwith a thin teflon-coated glass fiber sheet. The fiberglass sheetsshould lap over the inner and outer sides and over the end sides and theedges can be taped, glued or stapled. The top covering of the footing isdesignated in FIG. 2 by reference numeral 32.

The balsa wood footings are retained in position by means of retainingbars 34 and 36 which are suitable angle irons welded to subfloor 12.These retaining bars restrict the sideways expansion of the balsawoodunder the compressive load. Located at intervals along the length of thefoundation are shear key bars 38 and 40. Bar 38 is welded to subfloor l2and shear bar 40 is welded to tank floor 16. Pairs of these shear brsare provided transverse to the course of the foundation at intervalstherealong. These bars may be comprised of any suitable barstockmaterial.

The sliding bearing 24 is comprised of a relatively thin layer of balsawood with its grain oriented horizontally, i.e. perpendicular to thedirection of loading, so that the load is across the grain. The layermay be comprised of strips glued to one another. The layer should beprecompressed under conditions which prevent lateral spreading,conditions which obtain in service, since each piece of balsa abuts itsneighbor and has no room to spread sideways. The layer 24 is coveredwith a suitable cryogenic lubricant 25 at its bottom and is adhered tothe lower surface of the concrete wall 28. The balsa layer slides onfloor 16 when the prestressing operation forces wall 26 to moveinwardly. Once wall 26 is prestressed, its liner 28 is welded to thesteel floor plate 16 and the flat grain balsa wood 24 then acts as acushioning element.

With reference to FIG. 5, reference numeral 50 refers to a compositefooting of balsa wood, each block having its grain oriented vertically.Blocks are glued to one another as described above, and all surfaces ofthe balsa wood are sealed with fiberglass as described above. A plywoodpanel 52 is glued with cryogenic adhesive to the top surface of thebalsa wood and a similar plywood panel 54 is provided on the lowersurface. The purpose of the plywood panels 52 and 54 is to provideincreased resistance against tangential forces applied to thefoundation.

With reference to FIG. 6, reference numeral 60 refers to a compositefooting of balsa wood, softwood or hardwood, consisting of an assemblyof numerous small blocks, their axes oriented in the vertical, radialand tangential directions. The resulting composite is of approximatelyequal strength in all three principal directions, vertical, radial andtangential. lf superior strength is required in any of these directions,a higher percentage of end grain blocks is used in this particulardirection. Thus, a wooden structure can be created to suit given loadingand stress conditions.

What is claimed is:

l. in a cryogenic tank construction including a subfloor, a tank floorand a wall supported above said tank floor, the improvement whichcomprises:

a. a foundation comprised of wood positioned between said floor and saidsubfloor, said wood having its grain oriented parallel to said wall;

b. restraining means for said wood positioned along the sides of saidfoundation and fastened to said subfloor, running essentially parallelto the course of said foundation; and

c. restraining shear key bars for said wood positioned, at intervals,transversely to the course of said foundation and attached respectivelyto said tank floor and said subfloor, for preventing relative tangentialmovement of said foundation and wall.

2. In a cryogenic tank construction as recited in claim 1, the furtherimprovement which comprises coating said wood with fiberglass andcovering the upper surface thereof with teflon-coated glass fiber sheet.

3. In a cryogenic tank construction including a tank floor and a wallsupported thereon and wherein said wall is free to move radially inwardunder the influence of prestressing forces thereon, the improvementwhich comprises:

a slide bearing situated between the bottom surface of said wall and thetop surface of said tank floor, said bearing comprising a thin layer ofwood its grain oriented horizontally, said wood being attached byadhesive to said wall and having a cryogenic lubricant coating on itslower surface.

4. In a cryogenic tank including a steel subfloor, a steel tank floorand a prestressed concrete wall supported above said tank floor, theimprovement which comprises:

a. a foundation comprised of wood positioned between said floor-and saidsubfloor, said wood being comprised of a plurality of select blocksadhered to one another with a cryogenic adhesive and having a coating offiberglass thereon;

b. restraining bars for said wood positioned along the sides of saidfoundation and welded to said steel subfloor, said bars runningessentially parallel to the course of said wall;

c. restraining shear key bars for said wood positioned, at intervals,transversely to the course of said foundation and attached respectivelyto said tank floor and said subfloor, for preventing relative tangentialmovement of said foundation and wall; and

d. a slide bearing situated between the bottom surface of said wall andthe top surface of said tank floor, said bearing comprising a thin layerof precompressed balsa wood or softwood, its grain orientedhorizontally, said balsa or softwood being attached by adhesive to saidwall and having a cryogenic lubricant coated on its lower surface.

5. In a cryogenic tank construction including a subfloor, a tank floorand a wall supported above said tank floor, the improvement whichcomprises:

a. a foundation comprised of wood positioned between said floor and saidsubfloor, said foundation comprising select blocks being oriented in thevertical, radial and tangential directions with respect to saidfoundation;

b. restraining means for said wood positioned along the sides of saidfoundation and fastened to said subfloor, running essentially parallelto the course of said foundation; and

c. restraining shear key bars for said wood positioned, at intervals,transversely to the course of said foundation and attached respectivelyto said tank floor and said subfloor, for preventing relative tangentialmovement of said foundation and wall.

6. In a cryogenic tank including a steel sub-floor, a steel tank floorand a prestressed concrete wall sup ported above said tank floor, theimprovement which comprises:

a. a foundation comprised of wood positioned between said floor and saidsubfloor, said foundation comprising select blocks being oriented in thevertical, radial and tangential directions with respect to saidfoundation;

b. restraining bars for said wood positioned along the sides of saidfoundation and welded to said steel subfloor, said bars runningessentially parallel to the course of said wall;

c. restraining shear key bars for said wood positioned, at intervals,transversely to the course of said foundation and attached respectivelyto said tank floor and said subfloor, for preventing relative tangentialmovement of said foundation and wall; and

d. a slide bearing situated between the bottom surface of said wall andthe top surface of said tank floor, said bearing comprising a thin layerof precompressed balsa wood or softwood, its grain orientedhorizontally, said balsa or softwood being attached by adhesive to saidwall and having a cryogenic lubricant coated on its lower surface.

7. In a cryogenic tank, an improved construction comprising the elementsof: a foundation comprised of wood oriented with its grain in thevertical direction, said wood being coated with fiberglass and havingthe upper surface thereof covered with teflon-coated glass fiber sheet.

1. In a cryogenic tank construction including a subfloor, a tank floorand a wall supported above said tank floor, the improvement whichcomprises: a. a foundation comprised of wood positioned betwEen saidfloor and said subfloor, said wood having its grain oriented parallel tosaid wall; b. restraining means for said wood positioned along the sidesof said foundation and fastened to said subfloor, running essentiallyparallel to the course of said foundation; and c. restraining shear keybars for said wood positioned, at intervals, transversely to the courseof said foundation and attached respectively to said tank floor and saidsubfloor, for preventing relative tangential movement of said foundationand wall.
 2. In a cryogenic tank construction as recited in claim 1, thefurther improvement which comprises coating said wood with fiberglassand covering the upper surface thereof with teflon-coated glass fibersheet.
 3. In a cryogenic tank construction including a tank floor and awall supported thereon and wherein said wall is free to move radiallyinward under the influence of prestressing forces thereon, theimprovement which comprises: a slide bearing situated between the bottomsurface of said wall and the top surface of said tank floor, saidbearing comprising a thin layer of wood its grain oriented horizontally,said wood being attached by adhesive to said wall and having a cryogeniclubricant coating on its lower surface.
 4. In a cryogenic tank includinga steel subfloor, a steel tank floor and a prestressed concrete wallsupported above said tank floor, the improvement which comprises: a. afoundation comprised of wood positioned between said floor and saidsubfloor, said wood being comprised of a plurality of select blocksadhered to one another with a cryogenic adhesive and having a coating offiberglass thereon; b. restraining bars for said wood positioned alongthe sides of said foundation and welded to said steel subfloor, saidbars running essentially parallel to the course of said wall; c.restraining shear key bars for said wood positioned, at intervals,transversely to the course of said foundation and attached respectivelyto said tank floor and said subfloor, for preventing relative tangentialmovement of said foundation and wall; and d. a slide bearing situatedbetween the bottom surface of said wall and the top surface of said tankfloor, said bearing comprising a thin layer of precompressed balsa woodor softwood, its grain oriented horizontally, said balsa or softwoodbeing attached by adhesive to said wall and having a cryogenic lubricantcoated on its lower surface.
 5. In a cryogenic tank constructionincluding a subfloor, a tank floor and a wall supported above said tankfloor, the improvement which comprises: a. a foundation comprised ofwood positioned between said floor and said subfloor, said foundationcomprising select blocks being oriented in the vertical, radial andtangential directions with respect to said foundation; b. restrainingmeans for said wood positioned along the sides of said foundation andfastened to said subfloor, running essentially parallel to the course ofsaid foundation; and c. restraining shear key bars for said woodpositioned, at intervals, transversely to the course of said foundationand attached respectively to said tank floor and said subfloor, forpreventing relative tangential movement of said foundation and wall. 6.In a cryogenic tank including a steel sub-floor, a steel tank floor anda prestressed concrete wall supported above said tank floor, theimprovement which comprises: a. a foundation comprised of woodpositioned between said floor and said subfloor, said foundationcomprising select blocks being oriented in the vertical, radial andtangential directions with respect to said foundation; b. restrainingbars for said wood positioned along the sides of said foundation andwelded to said steel subfloor, said bars running essentially parallel tothe course of said wall; c. restraining shear key bars for said woodpositioned, at intervals, transversely to the course of said foundationand attached respectively to said tAnk floor and said subfloor, forpreventing relative tangential movement of said foundation and wall; andd. a slide bearing situated between the bottom surface of said wall andthe top surface of said tank floor, said bearing comprising a thin layerof precompressed balsa wood or softwood, its grain orientedhorizontally, said balsa or softwood being attached by adhesive to saidwall and having a cryogenic lubricant coated on its lower surface.
 7. Ina cryogenic tank, an improved construction comprising the elements of: afoundation comprised of wood oriented with its grain in the verticaldirection, said wood being coated with fiberglass and having the uppersurface thereof covered with teflon-coated glass fiber sheet.